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low hum on monitors

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low hum on monitors

Postby kinglouis » Wed May 08, 2019 10:49 am

hi all,

I'm getting a low hum from my Adam A7Xs when using my mics. It only happen when the gain is turned up on them. I am using a 414 B-uls, re20, sE4400, sm57s and sE4s running through an Apollo 8. The cables are new and they are running through a new stage box.

The hum happens with all mics, phantom powered or not, and increases with the gain on the apollo, so that by around 30dB on the apollo the sound is almost unusable from the hum.

Is there a check list of how to go through the gear?

Many thanks,

Alex
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kinglouis
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Re: low hum on monitors

Postby Hugh Robjohns » Wed May 08, 2019 11:03 am

There are several possibilities.

If the hum is audible when you're monitoring the mics live over speakers, it could be acoustic feedback.

If it's there when you're monitoring on headphones it could be either a ground-loop problem, or it could be induced electromagnetic pickup from mains getting into the mics or, more likely, the mic cables/snake.

It will take some careful and painstaking experimentation to find out the cause, and I'd start by listening to the preamp on its own, without any mics plugged in to see how clean it is and whether you have a ground loop between preamp and monitors/computer. Ideally you should use a shorting plug in the preamp input (150 Ohms wired between pins 2 and 3). If you don't use the shorting plug the preamp may well pick up interference that it wouldn't in normal use and give misleading results. If you don't have or can't make a shorting plug, connect a capacitor mic, but leave phantom turned off.

If that is clean you don't have a ground-loop between preamp and other gear... so you can progress to a single mic cable plugged into the preamp, with the shorting plug in place of the mic to see if the cable is picking up local EM interference from nearby equipment or the mains distribution within the room. I've come across rooms before where the mains distribution runs in a complete ring around the room, inducing hum into everything!

If the mic cable sounds clean, progress to a single mic lead plugged into the snake with the snake plugged into the preamp. I've come across problems before with snakes/stageboxes where the stagebox inadvertently links a studio ground back to the control room ground creating ground-loop hums...

Once you know where the hum is getting in, you can find a way of resolving it, but you have to know where it's getting in first!

H
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Re: low hum on monitors

Postby kinglouis » Wed May 08, 2019 11:43 am

That's exactly the trouble shooting system I was looking for, thank you very much Hugh, I'm very grateful.

Alex
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